在这个Clojure doto宏示例中打印关键字的目的是什么？

Question

I've been looking at this Clojure doto macro example from ClojureDocs , and I can't figure out what the purpose of the :printed keyword in the final println .

When I enter the example in a REPL, it prints out the HashMap as I would expect, just with a :printed displayed after the HashMap:

user=> (doto (java.util.HashMap.) (.put "a" 1) (.put "b" 2) (println :printed))
#<HashMap {b=2, a=1}> :printed
{"b" 2, "a" 1}

I figured println needed a placeholder so that it knows to wait for something coming from the doto macro. So I tried seeing what I'd get if I omitted :printed :

user=> (doto (java.util.HashMap.) (.put "a" 1) (.put "b" 2) (println))
#<HashMap {b=2, a=1}>
{"b" 2, "a" 1}

This one prints the same thing, but makes the HashMap without a :printed alongside it. Given this result, shouldn't the doto example give something like this:

#<HashMap {b=2, a=1}>
{"b" 2, "a" 1} :printed

What is the :printed keyword doing?

Answer 1

:printed simply adds " :printed" to the string printed by println . It does not affect the hash-map.

(println "Bingo" :printed)
=> Bingo :printed